2-line luminance-chrominance (YC) separation devices are increasingly drawing attention as inexpensive means for accurately separating luminance (Y) and chrominance (C) signals.
FIG. 11 is a block diagram of a 2-line YC separation device disclosed in Japanese Laid-open Patent Application No. H1-117494.
The 2-line YC separation device in FIG. 11 comprises a one horizontal scanning period delay circuit (1HDL) 2202, a subtractor 2203, another subtractor 2204, a band-pass filter (BPF) 2205, a switch circuit 2206, a chrominance subcarrier frequency trap circuit (T) 2207, a switch circuit 2208, a low-pass filter (LPF) 2211, a YL-correlation detector (YLC) 2212 (where the L component is the low-frequency luminance component), a band-pass filter (BPF) 2213, a chrominance detector (C) 2214, and an AND circuit 2215. The 2-line YC separation device separates a Y signal output 2209 and a chrominance signal output 2210 from a composite video signal 2201. The Y signal is assumed to consist of a Y signal component YH in the chrominance frequency band and a Y signal YL other than YH. The YL-correlation detector detects a line correlation (hereafter referred to as "correlation") between YL in a present line and YL in a signal delayed for one horizontal scanning period.
The operation of a 2-line YC separation circuit configured as above is described below.
First, a signal, after passing through the one horizontal scanning period delay circuit 2202, is subtracted from the current composite video signal. Then, this signal is made into a YL correlation output signal through the low pass filter circuit 2211. A detection output signal detected by the chrominance detector 2214 after filtering signals in the frequency band of the chrominance signal with the band-pass filter 2213. Both signals pass through the AND circuit 2215 and the output of AND circuit 2215 controls a comb-line filter.
More specifically, the switch circuit 2208 is turned on and, at the same time, the switch circuit 2206 is switched to the current video signal 2201 side only when the YL-correlation detector 2212 determines that i) there is no correlation, and ii) the chrominance detector 2214 detects a chrominance signal or the Y signal in the frequency band of the chrominance signal (YH). In other cases, for example, if the YL correlation detector 2212 determines that there is a correlation, or if the YL correlation detector 2212 determines that there is no correlation, but the chrominance detector 2214 detects no chrominance signal, the switch circuit 2208 is turned off and the switch circuit 2206 is switched to the band-pass filter 2205.
With the YC separation device as configured above, however, YC in an input signal pattern as illustrated in FIGS. 12A to 12J cannot be accurately separated. FIG. 12A is the case when a Y signal, in a vertical direction on a screen, whose frequency is equivalent to that of the chrominance subcarrier frequency, exists up to the 3H line and disappears thereafter.
Here, FIG. 12A is the composite video signal 2201; FIG. 12B is the output signal of the one horizontal scanning period delay circuit 2202; FIG. 12C is the output signal of the subtractor 2203; FIG. 12D is the output signal of the low-pass filter circuit 2211; FIG. 12E is the output of the YL-correlation detector 2212; FIG. 12F is the output of the chrominance detector 2214; FIG. 12G is the chrominance signal output from the chrominance output terminal 2210; and FIG. 12H is the Y signal output 2209 from the Y signal output terminal. Since the output signal of the band-pass filter 2205 is the result of filtering the frequency component of the chrominance subcarrier in the output signal of the subtractor 2203, the output signal of the subtractor 2203 and the band-pass filter 2205 become identical when a video signal such as the signal of FIG. 12A is input. The signal of FIG. 12E has the value "0" when there is a YL correlation, and "1" when there is no YL correlation. The signal of FIG. 12F has the value "1" when a chrorninance signal exists and "0" when there is no chrominance signal. FIG. 12I shows the desirable chrominance signal to be output, and FIG. 12J shows the desirable Y signal to be output.
Looking at FIGS. 12E and 12F, there is no case when i) the YL-correlation detector 2212 does not detect a correlation, and ii) the chrominance detector 2214 detects the presence of the chrominance signal in lines of 1H, 2H, 3H, 4H, and 5H. Consequently, the switch circuit 2208 is turned off and the switch circuit 2206 is switched to the band-pass filter 2205. In this case, the Y signal output becomes the same as the result of subtracting the output signal of the band-pass filter 2205, which is equal to the output signal of the subtractor 2203 in FIG. 12C, from the current composite video signal in FIG. 12A. Therefore, a signal in FIG. 12H is output. The chrominance signal output is the output signal of the band-pass filter 2205 which is equal to the output signal of the subtractor 2203 in FIG. 12C, and therefore a signal in FIG. 12G is output.
This means that in a video image as illustrated in FIG. 12A in which the Y signal in a vertical direction on a screen (having a frequency equivalent to that of a chrominance subcarrier) exists up to the 3H line, and disappears thereafter, the Y signal displayed is as shown in FIG. 12H, and the chrorninance signal displayed is as shown in FIG. 12G. However, a desirable chrominance signal to be output is as shown in FIG. 12I and a desirable Y signal is as shown in FIG. 12J when the signal pattern shown in FIG. 12A is input.
As described above, conventional technology is incapable of separating YC signals correctly, and may result in erroneous display operation, such as degraded horizontal resolution, appearance of color where there should be no color, and appearance of the Y signal where the chrominance signal should be.